The heart includes four valves that serve to direct blood flow through the two sides of the heart. On the left (systemic) side of the heart are: (1) the mitral valve, located between the left atrium and the left ventricle, and (2) the aortic valve, located between the left ventricle and the aorta. These two valves direct oxygenated blood from the lungs through the left side of the heart and into the aorta for distribution to the body. On the right (pulmonary) side of the heart are: (1) the tricuspid valve, located between the right atrium and the right ventricle, and (2) the pulmonary valve, located between the right ventricle and the pulmonary artery. These two valves direct de-oxygenated blood from the body through the right side of the heart and into the pulmonary artery for distribution to the lungs, where the blood becomes re-oxygenated in order to begin the circuit anew.
All four of these heart valves are passive structures in that they do not themselves expend any energy and do not perform any active contractile function. They consist of moveable “leaflets” that open and close in response to differential pressures on either side of the valve. Any or all of these heart valves in a particular patient may exhibit abnormal anatomy and function as a result of congenital or acquired valve disease. Congenital valve abnormalities may be well-tolerated for many years only to develop into a life-threatening problem in an elderly patient, or may be so severe that emergency surgery is required within the first few hours of life. Acquired valve disease may result from causes such as rheumatic fever, degenerative disorders of the valve tissue, bacterial or fugal infections, and trauma.
The problems that can develop with valves can generally be classified into two categories: (1) stenosis, in which a valve does not open properly, and (2) insufficiency (also called regurgitation), in which a valve does not close properly. Stenosis and insufficiency may occur concomitantly in the same valve or in different valves. Both of these abnormalities increase the workload placed on the heart. The severity of this increased stress on the heart and the patient, and the heart's ability to adapt to it, determine the treatment options that will be pursued. In some cases, medication can be sufficient to treat the patient, which is the preferred alternative; however, in many cases defective valves have to be repaired or completely replaced in order for the patient to live a normal life.
The two general categories of valves that are available for implantation into the cardiac system are mechanical valves and bioprosthetic or tissue valves. Mechanical valves have been used for many years and encompass a wide variety of designs that accommodate the blood flow requirements of the particular location where they will be implanted. Although the materials and design features of these valves are continuously being improved, they do increase the risk of clotting in the blood stream, which can lead to a heart attack or stroke. Thus, mechanical valve recipients must take anti-coagulant drugs for life to lessen the potential for blood clot formation. Further, mechanical valves can sometimes suffer from structural problems that may force the patient to have additional surgeries for further valve replacement.
Bioprosthetic valves, which are sometimes also referred to as prosthetic valves, generally include both human tissue valves and animal tissue valves. Prosthetic heart valves are described, for example, in U.S. Patent Publication No. 2004/0138742 A1 (Myers et al.), the entire contents of which are incorporated herein by reference. The designs of these bioprosthetic valves are typically relatively similar to the design of the natural valves of the patient and advantageously do not require the use of long-term anti-coagulant drugs. Human tissue valves are typically not available in large quantities since they must be removed from deceased persons who have elected organ donation; however, because large numbers of animals are routinely processed at meat processing facilities, for example, animal tissue valves are more widely available for the patients who require valve replacement. The most common types of animal tissue valves used include porcine aortic valves, and bovine and porcine pericardial valves, some of which are incorporated with some type of a stent before implantation in a patient.
To simplify surgical procedures and reduce patient trauma, there has been a recent increased interest in minimally invasive and percutaneous replacement of cardiac valves. Percutaneous replacement of a heart valve does not involve actual physical removal of the diseased or injured heart valve. Rather, the defective or injured heart valve typically remains in position while the replacement valve is inserted into a catheter and delivered percutaneously via the vascular system to the location of the failed heart valve. There, the replacement valve is either expanded by the balloon or self-expands to compress the native valve leaflets against the ventricular outflow tract, anchoring and sealing the replacement valve. In the context of percutaneous, pulmonary valve replacement, U.S. Patent Application Publication Nos. 2003/0199971 A1 (Tower, et al.) and 2003/0199963 A1 (Tower, et al.), describe a valved segment of bovine jugular vein, mounted within an expandable stent, for use as a replacement pulmonary valve. As described in the articles “Percutaneous Insertion of the Pulmonary Valve”, Bonhoeffer, et al., Journal of the American College of Cardiology 2002; 39: 1664-1669 and “Transcatheter Replacement of a Bovine Valve in Pulmonary Position”, Bonhoeffer, et al., Circulation 2000; 102: 813-816, the replacement pulmonary valve may be implanted to replace native pulmonary valves or prosthetic pulmonary valves located in valved conduits. Other implantables and implant delivery devices also are disclosed in published U.S. Patent Application Publication No. 2003/0036791 A1 (Bonhoeffer et al.) and European Patent Application No. 1 057 460-A1. In addition, percutaneous heart valves for use as a replacement pulmonary valve are described in Assignee's co-pending U.S. Patent Application Publication No. 2006/0206202 A1 (Bonhoeffer et al.). Like the valves described by Tower et al., the heart valves of this co-pending application incorporate a valved segment of bovine jugular vein, which is mounted within an expandable stent.
There is, however, a continued need to be able to be able to provide a variety of different valve assemblies to accommodate the requirements of different patients, such as by providing stented valves that can be designed and customized for each individual patient.